The present invention relates to the field of telecommunications, and more particularly, to a method and apparatus for reducing the peak-to-average power ratio in a discrete multi-tone signal, such as in a discrete multi-tone transmitter or transceiver.
Asymmetric Digital Subscriber Line (ADSL) is a communications technology that operates over existing twisted-pair telephone lines between a central office and a residential or business location. It is generally a point-to-point connection between two dedicated devices. ADSL supports bit transmission rates of up to approximately 6 Mbps in the downstream direction (to a subscriber device at the home), but only 640 Kbps in the upstream direction (to the service provider/central office). ADSL connections actually have three separate information channels: two data channels and a POTS channel. The first data channel is a high-speed downstream channel used to convey information to the subscriber. Its data rate is adaptable and ranges from 1.5 to 6.1 Mbps. The second data channel is a medium speed duplex channel providing bi-directional communication between the subscriber and the service provider/central office. Its rate is also adaptable and the rates range from 16 to 640 kbps. The third information channel is a POTS (Plain Old Telephone Service) channel. The POTS channel is typically not processed directly by an ADSL modem. The POTS channel operates in the standard POTS frequency range and is typically processed by standard POTS devices after being split from the ADSL signal.
The American National Standards Institute (ANSI) Standard T1.413, the contents of which are incorporated herein by reference, specifies an ADSL standard that is widely followed in the telecommunications industry. The ADSL standard specifies a modulation technique known as Discrete Multi-Tone modulation.
Discrete Multi-Tone (DMT) uses a large number of subcarriers spaced close together. Each subcarrier is modulated during training using Quaternary Phase Shift Keying, or QPSK. Training typically consists of adjusting to existing conditions in the communications connection, such as amplitude response, delay distortions, time recovery, and echo characteristics. During normal data transmission mode, the modulation used in ADSL is Quadrature Amplitude Modulation (MQAM). The data bits are mapped to a series of symbols in the I-Q complex plane, and each symbol is used to modulate the amplitude and phase of one of the multiple tones, or carriers.
In some ADSL transceivers, the symbols are used to specify the magnitude and phase of a subcarrier, where each subcarrier frequency corresponds to the center frequency of the xe2x80x9cbinxe2x80x9d associated with a Discrete Fourier Transform (DFT). The modulated time-domain signal corresponding to all of the subcarriers can then be generated in parallel by the use of well-known DFT algorithms called Inverse Discrete Fourier Transforms (IDFT). There are many well-known forms of the DFT and IDFT, often referred to generically as fast Fourier transforms (FFT) and inverse fast Fourier transforms (IFFT).
The symbol period in ADSL modems is relatively long compared to single carrier systems because the bandwidth available to each carrier is restricted. However, a large number of symbols is transmitted simultaneously, one on each subcarrier. The number of discrete signal points that may be distinguished on a single carrier is a function of the noise level. Thus, the signal set, or constellation, of each subcarrier is determined based on the noise level within the relevant subcarrier frequency band. The appropriate loading of each carrier is determined during initial training and analysis periods.
Because the symbol time is relatively long and is preceded by a guard band, intersymbol interference is a less severe problem than with single carrier, high symbol rate systems. Furthermore, because each carrier has a narrow bandwidth, the channel impulse response is relatively flat across each subcarrier frequency band. The DMT standard for ADSL, ANSI T1.413, specifies 256 subcarriers, each with a 4.3125 kHz bandwidth. Each subcarrier can be independently modulated from zero to a maximum of 15 bits/sec/Hz. This allows up to around 60 kbps per tone. DMT transmission allows modulation and coding techniques to be employed independently for each of the subchannels.
The subchannels overlap spectrally, but as a consequence of the orthogonality of the transform, if the distortion in the channel is mild relative to the bandwidth of a subchannel, the data in each subchannel can be demodulated with a small amount of interference from the other subchannels. For high-speed wide-band applications, it is common to use a cyclic-prefix at the beginning, or a periodic extension appended at the end of each symbol to maintain orthogonality. Because of the periodic nature of the FFT, no discontinuity in the time-domain channel is generated between the symbol and the extension. It has been shown that if the channel impulse response is shorter than the length of the periodic extension, subchannel isolation is achieved.
Signal processing is typically performed after the signal waveform is sampled. Processing associated with ADSL modems often includes echo cancellation, equalization, and DMT modulation/demodulation.
Although DMT has been adopted by standards organizations for ADSL modems, DMT has some disadvantages. One of the most significant disadvantages is its large time-domain PAR (Peak-to-Average power Ratio). A large PAR requires higher resolution digital-to-analog conversion to avoid clipping of the signal, which results when a DMT signal exceeds the dynamic range of the DAC (Digital-to-Analog Converter). As resolution requirements increase, the cost of a suitable DAC increases. Similarly, when a transmitted signal having a large PAR is reflected back into the receiving path (a local echo), it causes similar problems for the ADC (Analog-to-Digital Converter). A large PAR requires higher resolution analog-to-digital conversion to avoid signal-clipping. Typically, the cost for increased resolution in an ADC is even greater than for increased resolution in a DAC. As a result of these high resolution DACs and ADCs, ADSL modems for subscriber devices, e.g., personal computers and other remote terminals, are significantly more expensive than traditional analog POTS-type modems.
One prior solution for PAR reduction is described in U.S. Pat. No. 5,787,113. This first solution generally involves limiting and truncating a DMT signal before digital-to-analog conversion occurs. An echo cancellation scheme is described to cancel noise introduced by the limiting process. However, the noise will most likely be a wide-band or white noise, which is difficult to cancel, compared to noise generated from a smaller number of frequencies.
A second prior solution for PAR reduction is described in U.S. Pat. No. 5,835,536 (inventors Michael May, Terence Johnson, and Matthew Pendleton). Variations of this second solution are discussed in Jose Tellado, John Cioffi, and Richard Stuart, xe2x80x9cPAR reduction in Multicarrier Transmission Systems,xe2x80x9d ITU Contributions, D.150, Geneva, Feb. 9-20, 1998, and in A. Gatherer and M. Polley, xe2x80x9cControlling Clipping Probability in DMT Transmission,xe2x80x9d The 31st Asilomar Conference on Signals, Systems, and Computers, Nov. 1997. This second solution generally involves using tones within the DMT transmission band to generate a magnitude adjusting symbol to add to the time-domain DMT signal to be transmitted. As the number of tones used to generate the magnitude adjusting signal increases, the reduction in PAR improves. The tones used for PAR reduction in this second solution are often tones within the transmission band that may be insufficient for supporting data transmission. However, tones used for PAR reduction cannot be used for data transmission. Therefore, if it is desired to use more tones to achieve improved PAR reduction, tones that might otherwise be used for data transmission are instead used for PAR reduction. Hence, there is a loss in data rate due to the unavailability of these tones for data transmission.
Needed is a method and/or apparatus for reducing PAR in a DMT device that results in a minimal loss of data rate. The method and/or apparatus should have a minimal effect on the signal to be transmitted, and should allow any effects the PAR reduction introduces to a received signal to be substantially cancelled.
In accordance with preferred embodiments of the present invention, some of the problems associated with PAR reduction are overcome. Methods and apparatus for PAR reduction in a DMT signal are provided.
One aspect of the invention includes a method for reducing peak-to-average ratio in a discrete multi-tone signal. First, a shaping signal is generated. The shaping signal includes frequency bands located outside a transmission band associated with a discrete multi-tone signal. The discrete multi-tone signal may be a signal that is to be transmitted across a transmission medium, for example. The shaping signal is added to the discrete multi-tone signal, resulting in a modified discrete multi-tone signal. This modified signal may then be further processed and transmitted.
In one preferred embodiment of the invention, after the signal has been modified by the shaping signal, the modified signal is converted to an analog signal, which is then filtered to suppress a shaping component associated with the shaping signal.
In another preferred embodiment of the invention, echo cancellation may be performed on a second discrete multi-tone signal, such as a signal received by a transceiver from a communications medium.
In a second aspect of the invention, a method for reducing peak-to-average power ratio in a transmit signal in a discrete multi-tone transceiver is provided. A discrete multi-tone signal is interpolated to produce an interpolated discrete multi-tone signal. Next, a peak magnitude associated with either the original signal or the interpolated signal is compared to a pre-determined magnitude parameter. If the peak magnitude exceeds the pre-determined magnitude parameter, then a shaping signal is added to the interpolated discrete multi-tone signal to produce a modified signal. In this aspect of the invention, the shaping signal consists entirely of tones located outside a transmission band allocated to the discrete multi-tone signal. Feedback tones from the shaping signal that may be introduced to a receive signal within the discrete multi-tone transceiver may be cancelled, such as by subtracting an echo cancellation parameter from the receive signal.
Another aspect of the invention includes a discrete multi-tone transmitter. The discrete multi-tone transmitter includes an Inverse Fast Fourier Transform unit, an interpolation unit, a shaping signal generator that uses tones located outside a transmission band, an adder, a digital-to-analog converter, one or more filters for suppressing the shaping signal, and an interface.
In a preferred embodiment of the invention, the transmitter is part of a transceiver that further includes an analog-to-digital converter and an echo cancellation unit. In this embodiment, the echo cancellation may be substituted for the one or more filters for suppressing the shaping signal.